The present invention relates to plasma display systems and, more particularly, to a method of light intensity level compensation in accordance with the field frequency of an input video signal.
In a plasma display, its drive input voltage and light emission output are non-linearly related, so that it cannot display an input video signal as images with analog light intensity level gradations. Accordingly, the light "on" period is divided into divisions, and gradations are provided by whether or not light is to be "on" in each division, that is, in terms of the number of divisions, in which light is "on". For example, a sub-field scheme as shown in FIG. 6(a) is used, in which one field is constituted with a plurality of sub-fields (SF) with different light "on" period ratios (proportional to the light intensity level). The video signal is converted into digital data of bits corresponding in number to the number of the sub-fields in the field, and this data is used to cause the light emission from corresponding image elements in an appropriate sub-field (or a plurality of appropriate sub-fields).
As an example, a plasma display will now be considered, which uses an AC discharge memory type plasma display panel having a plurality of photo-cell scan electrodes, sustained discharge electrodes paired with and formed on the same plane as the respective photo-cell scan electrodes, a plurality of data electrodes crossing the photo-cell scan and sustained discharge electrodes, and a plurality of photo-cells formed for display at the intersections of the photo-cell scan and sustained discharge electrodes and the data electrodes. The sub-field scheme which is adopted in this example, is of a discrete photo-cell scan/sustained discharge type, in which one field is constituted by n sub-fields with video signal light intensity level ratios of 2.sup.n-1 :2.sup.n-2, . . . , 2:1. Each sub-field has a photo-cell scan period P for determining the period of "on" or "off" of the photo-cells, a sustained discharge period S for repeatedly discharging on the basis of selective discharge in the photo-cell scan period, and a preliminary discharge period for preliminary discharge prior to the photo-cell scan period. A light emission sequence is arranged by setting progressively reduced sustained discharge pulse numbers in the sustained discharge periods of the successive sub-fields and thus setting a fixed basic light intensity level such that the light intensity level ratios of the successive sub-fields are 2.sup.n-1 :2.sup.n-2, . . . , 2:1. With such light intensity level combination of the n sub-fields, a video signal is displayed as multiple gradation display.
In this type of prior art plasma display, a sub-field sequence is formed with a certain fixed field frequency as a reference frequency. This arrangement permits display of video signal at a field frequency lower than the reference frequency. In this case, however, the drive period is reduced compared to the field period, thus resulting in a pause period from the instant of the end of the drive operation in a field till the instant of the start of that in the next field. This means a variation of the apparent drive frequency, i.e., a variation of the input video signal field frequency, thus resulting in a light intensity level variation.
The prior art plasma display has a problem that the light intensity level of display is varied with a variation of the field frequency of the video signal to be displayed because of the sub-field sequence configuration, which is based on the maximum field frequency, i.e., the upper limit field frequency, of video signal that can be displayed.
The field frequency of video signal which is actually displayed, ranges from about 50 to about 75 Hz. When a video signal of field frequency 50 Hz is displayed with a sub-field sequence which is formed with a reference field frequency of 75 Hz, the light intensity level is reduced by about 30%, and the display is visually recognized to be darker.
The reason for this is as follows. The prior art plasma display adopts a fixed sub-field sequence for the driving, which is formed by using, as a reference frequency, the vertical sync signal frequency (which is 60 Hz in the prior art example shown in FIG. 6(a)), i.e., the highest frequency in video signal capable of being displayed on the plasma display. With such a fixed sub-field sequence, a field frequency change in such case as when the input video signal is switched, does not cause any change in the sustained light emission period. Such a fixed sub-field sequence results in an inter-field drive pause period R shown in FIG. 6(b) for instance, when the displayed video signal is switched to one at a field frequency lower than the reference field frequency. The apparent drive frequency is therefore reduced to reduce the light intensity level of display.
In the display adopting a sub-field scheme for multiple gradation, using a fixed sub-field sequence for displaying different kinds of video signals, results in video signal field frequency variations to vary the light intensity level.